Practical amplifiers fall short of ideal amplification due to at least two limiting factors: noise and nonlinearities. Noise added to an amplified signal degrades the "quality" of low-level signals while device nonlinearities distort large-amplitude signals. The useful range over which an active device may amplify power is called the dynamic range. The lowest level signal that can be amplified is governed by internal device noise. The highest level signal that can be amplified is determined by device nonlinearities which cause distortion. In addition to minimizing the noise and maximizing the linearity of the device, it is also desirable to minimize the dc power required to accomplish this. This requirement is often expressed in a figure of merit equal to the ratio of the maximum microwave output power (at a specified level of distortion) to the applied DC power. The conventional method of specifying the level of distortion for this figure of merit is called the output intercept point of third order products, or simply OIP3. The OIP3 method applies two input signals separated only slightly in frequency, and of substantially equal, but adjustable, power. A plot is made of both the fundamental frequency output power and the power in the third order intermodulation product versus the input power and a linear extrapolation is made of these two plots. The point where these two extrapolations intersect is the OIP3 amplitude, which is read in dBm from the output power (ordinate) axis.
Attempts to improve amplifier dynamic range fall into at least two categories: circuit techniques and intrinsic improvements in the active device itself. Circuit techniques, such as feed forward or predistortion, are effective but can result in complicated and power consuming circuits. Intrinsic device improvements in the area of microwave FETs have been centered on schemes to make a device with linear transfer characteristics, i.e. an FETwith constant transconductance, g.sub.m =.DELTA.I.sub.DS /.DELTA.V.sub.GS (transconductance equals the change in drain current divided by the change in gate voltage).
Williams and Shaw (see "Graded Channel FET's: Improved Linearity and Noise Figure", IEEE Transactions on Electron Devices, vol. ED-25, no. 6, pp 600-605, June 1978) in their theoretical study of the subject emphasized using a special doping profile to maintain a constant transconductance at all gate voltages. The structure used by Chu, et al (see "A Highly linear MESFET", IEEE-MTT-International Microwave Symposium Digest, pp 735-728, 1991), while achieving constant g.sub.m, is very complex and the gate region is difficult to fabricate reproducibly using conventional etching techniques. Applicants have previously disclosed a simpler structure than that of the aforementioned prior an for obtaining similar results (See Ikalainen and Witkowski, "High-Dynamic Range Microwave FET", Electronics Letters, vol. 27. no. 11, pp 945-6, May 23, 1991).